Process for densifying felts with formamide



3,057,036 PRGCESS FUR DENEFYENG FELTS WITH FGRMAMEDE Nathan H. Koenig, Berkeley, Calif, assignor to the United States of America as represented by the Secretary of Agriculture No Drawing. Filed Sept. 27, 1960, Ser. No. 58,863

5 Claims. (Cl. 28-76) (Granted under Title 35, US. Code (1952), see. 266) A non-exclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This invention relates to and has as its primary object the provision of novel methods for increasing the density and imparting other desirable changes in the properties of wool-containing felts by chemical treatment. Further objects and advantages of the invention will be evident from the following description wherein parts and percentages are by weight, unless otherwise specified.

In the conventional preparation of felts, Webs of carded wool, with or without other fibers, are deposited one over the other to form loose batts. These batts are moistened, steamed, treated with a lubricant such as soap, and subjected to vibratory pressure to compact the batts. The compacted batts are then subjected to repeated pounding, rolling, and kneading to interlock the fibers into a dense sheet form. As this mechanical action, generally referred to as fulling, is continued the felt becomes increasingly dense. In preparing soft felts the mechanical action may be applied for relatively short times of about minutes; for preparing hard sheet felts the mechanical action must be continued for 12 hours or more. Hard felts are often required for particular applications, for example for polishing wheels; artificial limbs; cushioning pads for looms, presses, and other machinery; clutches; brakes, etc.

In accordance with the invention, felts are hardened or densified by a chemical action instead of mechanical action. A particular advantage of the process of the invention is that elimination of the extended mechanical treatment results in a substantial saving in operating costs. Another advantage is that the time required to achieve a desired degree of hardness is less than With mechanical treatment. Moreover, the process of the invention requires no specialized equipment and is outstanding in its simplicity and effectiveness.

The objects of the present invention are attained by impregnating a felt with formamide and applying heat. The heatin is continued until the felt develops the desired degree of hardness.

When the felt is heated in the presence of formamide, the individual wool fibers contract or shrink, this causing the entire mass of fibers to occupy a decreased volume, thus forming a denser mass. Where the felt contains other fibers in addition to wool, the shrinkage of the wool 'fibers naturally draws these fibers together since all the fibers of the mass are matted and entangled. Generally, under any specified conditions of temperature, time, etc., a greater densification will occur with felts of greater proportion of wool fibers. Although the theory of the process is imperfectly understood, it has been determined that there is no significant increase or decrease in weight as a result of the process of the invention. It is therefore believed that the formamide acts as a catalyst in promoting a chemical rearrangement, rather than adding to the Wool molecules. Thus certain chemical bonds in the wool molecule are disrupted and then re-formed into new patterns; as a result, the individual fibers contract. Although certain chemical changes are believed to take place, it is to be noted that wool is insoluble in formamide under the conditions described herein and the Wool fibers do not fuse or otherwise lose their identity in the process of the invention.

As noted above, the process of the invention results in increasing the density of the original felt. Depending on the conditions applied, the decrease in volume may be as low as 5% or as high as about 50%. In addition to increased density, the products exhibit increase in hardness and splitting resistance. These desirable changes are believed to be caused by the increased degree of fiber interlocking in the treated felts.

It has been observed that when commercial felts are hardened by the process of the invention, the dimensional decrease is mainly in the length and width of the sheet. There is usually little change in thickness. This is believed to be due to the way the felts are ordinarily made by superimposing webs of carded fibers. As a result there is a lesser degree of interlocking of fibers in a plane normal to the surface of the sheet than in planes parallel to the surface. If .it is desired to attain a greater reduction in thickness during the formamide treatment, the felts may be given a pre-treatment to attain a greater degree of interlocking of fibers in the plane normal to the surface. This can be accomplished by applying the known process of needling; that is, by repeatedly forcing barbed needles through the sheet.

In one modification of the invention, the felt to be hardened is first impregnated with formamide, then heated. The formamide may be applied to the felt as such or dissolved in a volatile inert solvent such as water, methanol, ethanol, propanol, isopropyl alcohol, etc. Application of the compound (or solution thereof) to the felt may be by any conventional technique as immersion, spraying, or by the use of lick rolls. Since formamide acts as a catalyst rather than as a reagent, the proportion applied to the felt is not cricical. For example, the proportion of formamide may be as low as 10% based on the Weight of the felt. Usually, however, a larger proportion of the compound is used to compensate for loss of the compound by vaporization during the heating step. The use of excessive amounts of formamide does not do any harm and the residue in the product is easily removed after heating by extraction.

In the heating step the formamide-impregnated felt is heated at a temperature which may range from about to about C. The heating may be in an oven, by subjection to heated platens, by irradiation with infrared rays, etc. The time of heating will vary depending on the conditions used, particularly temperature, and on the degree of densification desired. Higher temperatures are conducive to increased rate of densification. Also, as the heat treatment is continued, the product becomes increasingly dense. Heating times may be anywhere from 5 minutes to 12 hours, depending on the temperature selected and the degree of densification desired. To prevent loss of formamide by vaporization, the felt may be heated while contained in a sealed vessel such as a retort, autoclave, or the like.

Although it is generally preferred to carry out the process of the invention by first applying formamide to the felt and then heating, the formamide may be applied simultaneously with heating. Thus, for example, the felt may be immersed in a pool of formamide maintained at the aforesaid temperature range (85-130" C.). The felt is held therein until the desired degree of densification is attained. Another plan is to expose the felt to hot vapors of formamide. Thus the felt may be suspended in a vessel containing at the base a suitable quantity of formamide. By applying heat to the base of the vessel, the formamide is vaporized and so distributed into the interstices of the felt. The vessel used in this procedure may be a pressure-tight autoclave, in which case the system may be maintained at a pressure lower than atmospheric so that the formamide will boil at a temperature within the desired range of 85l30 C. At normal pressure formamide boils at 193 C.

After the felt has been heated in the presence of formamide for a period sufficient to attain the desired increase in density, any residual formamide may be removed by extracting the felt with water, acetone, methanol, ethanol, propanol, isopropyl alcohol, or other volatile solvent for the compound.

The process of the invention may be applied to all-wool or part-wool felts. Generically then, the invention is applicable to wool-containing felts, by which expression is meant felts containing at least 20% by weight of wool.

The remaining fibers in the felt may be any natural or artificial textile fibers which are insoluble in formamide. Thus the non-wool components of the felt may be cotton; kapok; linen; natural silk; regenerated cellulose fibers produced by the viscose, cuprammonium, or nitro-cellulose process; jute; hemp; animal hair; glass fibers; asbestos, etc.

The invention may be utilized in various ways. For example, commercial grades of felts may be hardened to any desired density by application of the process of the invention. Another plan is to use the chemical process of the invention as a substitute for all or part of the conventional mechanical fulling action. Thus for example, batts of fibers may be subjected to fulling only to the extent of forming a soft felt and then the process of the invention is applied to form a hard felt therefrom.

A feature of the invention is that formamide is em ployed .as the sole active agent in densifying felts; no other felting or fulling agent is employed in applying the process of the invention. This feature of the invention, as well as the absence of mechanical action, adds to the simplicity of the instant process.

The invention is further demonstrated by the following illustrative examples:

Example I A sample of black, soft pad wool felt was obtained containing virgin wool, 83% reprocessed and reused wool, 7% cotton, and 5% miscellaneous fibers. Two strips were cut from this felt, each measuring cm. long by 4.7 cm. wide by 0.6 cm. thick. The strips were heated in a bath of formamide at 101 C. for the times indicated in the table below. The treated felts were washed in cold water for 2 hours to remove excess formamide. The felts were then squeezed out between rubber rolls to remove excess moisture and dried overnight under a stream of air. Specimens of the treated and untreated felt were tested for splitting resistance by the method ASTM D46l-53 using 1 inch strips of felt. The dimensional changes and splitting resistance are given below:

Increase Splitting Width, Length, Thickin resist- Sample em. cm. ness, density. ance for 1 cm. percent of inch strip,

original lbs.

Untreated 4. 7 10.0 0. 6 1. 4

Treated (1 hr. heating)--. 4. 3 9. 4 0. 6 14 2. 2

Treated (2 hr. hcating) 3. 8 8. 2 0. 6 36 3. 7

Example II Increase Splitting in resist- Satnple density, ance 1 Hardness percent of inch strip,

original lbs.

Untreated Treated (15 min.) Treated (30 min.)

Example 111 A sample of felt of the same starting felt as used in Example I was treated in a formamide bath at a temperature of 115 C. for 30 minutes. The treated felt was washed and dried as in Example I. The density increase and the results of the splitting resistance and hardness tests are tabulated below. There was no change in the sample thickness.

25% water for 20 minutes at 116 C. The sample was washed and dried as in Example I and then measured to determine the density increase. The results of the splitting resistance and hardness tests are tabulated below:

Increase Splitting in resist- Sample density, ance 1 Hardness percent of inch strip,

original lbs.

Untreated 1. 4 22 Treated 41 2. 8 40 Example V A 2 X 4" sample of medium pad, inch thick, white wool felt was heated for 5 minutes at 123 C. in a bath of formamide. The treated felt was washed and dried as in Example I. The density increase and hardness test results are tabulated below:

Increase in Sample density, Hardness percent of original Untreated 25 Treated 51 37 Example VI A 2-inch square sample of felt containing 77% wool and 23% rayon was heated with formamide at C. for 4.2 hours. The treated felt was extracted with hot acetone and air dried. The increase in density of the felt was 36%.

Example VII A Z-inch square sample of felt containing 40% wool, 40% rayon and 20% cotton was heated in formamide for 4.5 hours at 105 C. The treated felt was extracted with cold water followed by hot acetone and then air dried. The increase in density of the felt was 15%.

Having thus described the invention, what is claimed is:

1. A process for increasing the density, hardness, and splitting resistance of a wool felt which comprises impregnating a wool felt with formamide and heating the impregnated felt at a temperature about from 101 to 123 C. for a time requisite to increase the density, hardness, and splitting resistance of the wool, said time period ranging from 5 minutes at the upper end of said temperature range to 2 hours at the lower end of said temperature range.

2. A process for increasing the density, hardness, and splitting resistance of a wool felt which comprises impregnating a wool felt with formamide and heating the impregnated felt at about 101 C. for about 1 to 2 hours.

3. A process for increasing the density, hardness, and splitting resistance of a wool felt which comprises impregnating a wool felt with formamide and heating the impregnated felt at about 113 C. for about 15 to 30 minutes.

splitting resistance of a wool felt which comprises impregnating a wool felt with formamide and heating the impregnated felt at about 115 C. for about 30 minutes.

5. A process for increasing the density, hardness, and splitting resistance of a wool felt which comprises impregnating a Wool felt with formamide and heating the impregnated felt at about 123 C. for about 5 minutes.

References Cited in the file of this patent UNITED STATES PATENTS Hofiman July 5, 1955 OTHER REFERENCES Elod and Zahn: Journal of the Textile Institute, Ab-

4. A process for increasing the density, hardness, and 15 stracts, page A-327, July 1947. 

